Battery charger using a controlled scr to provide tapering charging characteristics

ABSTRACT

The battery charger comprises a transistorized system comprising a stepdown transformer coupled to a silicon-controlled rectifier, with the power output of the rectifier controlled by a sensing network in circuit with phase shift circuit means. A Zener diode is provided which furnishes a reference voltage applied across a voltage divider in the sensing circuit wherein no current can flow to a battery so long as the battery voltage equals the reference voltage. If the battery voltage drops due to a load or for some other reason, current will bias a transistor into a conduction condition to actuate the phase shift network. The latter is coupled to the silicon-controlled rectifier to gate the rectifier in a manner automatically to charge the battery at a rate inversely proportional to the amount of charge on the battery.

United States Patent GATE [72] Inventor Edmund Kidd 3,375,427 3/1968Magner et a1 321/5 Garfield Heights, Ohio 3,414,774 12/1968 Motta 320/31X [21] Appl, No. 022,123 2,714,188 7/1955 Scherer 323/6 X [22] FiledMar. 31,1970 3,241,053 3/1966 Woodley 323/22 X [45] Patented June 15,1971 3,300,704 1/1967 McMillen 320/61 [73] Assignee Crawford FittingCompany 3,363,163 1/ 1968 Nord et al.. 320/31 2 Ohio 3,382,425 5/1968Legatti 320/32 of appliauon Prima Examiner-J D Miller 603862 1966 nowabandoned Assista m Examiner-.lohn M. Gunther Attorney-Fay, Sharpe andMulholland [54] zgi w g azgig ABSTRACT: The battery charger comprises atransistorized CHARACTERISTICS system comprlsmg a stepdown transformercoupled to a 511 Chm, 1 mm H icon-controlled rectifier, with the poweroutput of the rectifier controlled by a sensing network in circuit withphase shift cir- UOS. u; cuit means A Zener is provided furnishes a320/32, 321/ 19, 323/6 reference voltage applied across a voltagedivider in the llll. Cl. t s t sensing circuit wherein no current canflow to a battery so [50] Field of Search 320/31, 32, l as he batteryvoltage equals the reference voltage. 1f the 40, BIG 2; 321/19; 323/6,22 battery voltage drops due to a load or for some other reason,

' 5C current will bias a transistor into a conduction condition to ac-[561 References Cited mate the phase shift network. The latter iscoupled to the silicon-controlled rectifier to gate the rectifier in amanner auto- UNITED STATES PATENTS matically to charge the battery at arate inversely proportional 3,310,729 3/1967 Burgess et a1 321/18 X tothe amount ofcharge on the battery.

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BATTERY CHARGER USING A CONTROLLED SCR TO PROVIDE TAPERING CHARGINGCHARACTERISTICS This is a Continuation of application Ser. No. 603,962,filed Dec. 22, 1966 by the same inventor.

The present invention relates to an automatic control system for abattery charger and more particularly to a system utilizing a phaseshift circuit to control an electronic rectifier of the type having agate electrode.

Battery operation of many types of electrical equipment has come intowide and varied use in present-day situations, and an automaticrecharging mechanism for batteries is generally desirable and sometimesmandatory.

The present invention provides a battery-charging circuit including anelectronic rectifier having a gate electrode which is controlled by aphase shift system circuit so as to vary the power output of theelectronic rectifier in inverse proportion to the charge on the battery.

Accordingly an object of the invention is to provide an automaticcontrol system for a battery-charging device.

A further object of the invention is to provide an automatic controlsystem of the latter type which is transistorized.

A still further object of the invention is to provide an automaticcontrol circuit of the above-mentioned type including an electronicrectifier controlled by a phase shift circuit so that the chargingcurrent is inversely proportional to the charge on the battery.

Other objects and advantages of the invention will be apparent from thefollowing description taken in conjunction with the accompanyingdrawings wherein the FIGURE is a schematic diagram of thebattery-controlled circuit of the invention.

As illustrated, a source of charging current for the battery controlcircuit may be supplied from conventional main powerlines and 12providing alternating current which is reduced to proper voltage andwhich is then introduced into the control circuitry. The current fromthe alternating current commercial powerlines may be passed to astepdown transformer Tl having a center-tapped secondary, with each legof the secondary being connected to rectifiers CR1 and CR2 therebyproviding for full-wave rectification of the transformer secondarycurrent. The resultant pulsating direct current flows to an electronicrectifier SCR, which may be a silicon-controlled rectifier which willnot conduct a current until the gate electrode thereof is triggered by apulse. The importance of the silicon-controlled rectifier lies in theease with which its conduction can be initiated and timed. Once the gateof the rectifier SCR is triggered, the rectifier will continue toconduct current from the anode to the cathode until the anode voltagedrops to near zero with reference to the cathode. With a 60-cycle supplysource of the anode, the voltage would drop to near zero as previouslydiscussed about 120 times per second due to the full-wave rectificationof rectifiers CR1 and CR2. Accordingly, the gate electrode is adapted tobe triggered 120 times per second, with the power output beingcontrolled by a phase shift method.

As diagrammatically illustrated in the drawing in the upper left-handcorner thereof, if the gate is triggered at the beginning of eachhalf-cycle, the power output would be maximum. If the pulse is delayed,the power output would correspondingly fall off to a lower value, sincethe current can now only flow during the remainder of each half-cycle,for example, as shown in the shaded portion of said pulse. If the directcurrent then flows through a limiting resistor R1, and is then appliedacross a Zener diode ZN.

The diode ZN will not conduct until the voltage applied thereacrossreaches the critical avalanche, or Zener point. As the input voltage isincreased above this Zener point, current will flow through the diode,keeping the voltage constant, providing a pulsating flat-topped wave asillustrated in the drawing, entitled Reference Voltage, which willremain constant regardless of line voltage fluctuations. Theseflat-topped waves are now the reference voltage, as shown in point A ofthe drawing.

The reference voltage is applied across a voltage divider comprisingresistor R5, potentiometer R6, and resistor R7.

Potentiometer R6 is set for 'the required full-charge battery voltage.Under these conditions, the flat-topped reference voltage is adapted toequal the battery voltage and, therefore,

i no current can flow through transistor 01. Should the battery pulse isdelayed further, a point will be reached where the involtage drop, dueto a load or leakage, the reference voltage will then be higher thanbattery voltage. This change in voltage will bias transistor 01 intoconduction, current-flowing base to emitter through protective lamp DLto the positive side of battery.

Since the flat-topped reference voltage waves are almost square waveswhen applied to the collector of Q1, the resultant amplified pulses areapplied to primary of transformer T3. The pulses are broadened out bycapacitor C1.

The secondary of transformer T3 is coupled to transistor Q2 causing itto conduct, permitting current to flow from its emitter through resistorR2, charging the capacitor C2, which forms the phase shift network. Whenthe voltage on capacitor C2 reaches the critical voltage, a unijunctiontransistor 03 will conduct, discharging capacitor C2 through the primaryof transformer T4, generating a pulse which is coupled into secondarywinding of transformer T4. The secondary of transformer T4, in turn, isconnected between the gate and cathode of the silicon-controlledrectifier SCR, causing it to conduct. The lower the battery voltage, thegreater will be the amplitude of the pulse on transistors 01 and Q2, andthe greater will be the current flow through the transistor Q2, therebycharging the capacitor C2 sooner, and causing unijunction transistor 03to fire closer to the beginning of each half-cycle, thus gatinglSCRsooner to put out more power.

Since alll the transistors have pulsating DC applied to their respectivecollectors of Q1 and Q2, and base of Q3, the whole phase shiftlnetworkis synchronized, or locked, thereby giving smooth control from zerocharge to full charge.

Resistor R4 leaks off any residual charge on capacitor C2 between eachhalf-cycle. Current-limiting control transformer T5 samples pulsating DCcharging current flowing through its primary to the battery, then stepsup the voltage in its secondary, potentiometer R9 being the secondaryload of transformer T5.

Potentiometer R9 is set for the required limit current. The resultant ACvoltage is rectified by CR3, charging capacitor C4, which in turn'biasestransistor 04 into conduction, which in turn leaks off some of thecharge on capacitor C2, causing it to charge slower (shifting phase)thereby delaying the firing of unijunction transistor 03, and cuttingdown the charging rate, should the load on the battery exceed thecharging rate of the charger.

Resistor R8 supplies protective bias for transistor Q4. Capacitor C3across the cathode-gate of the SCR bypasses any extraneous spikes orR.F. energy so as to prevent said energy from triggering the SCR.

Lamp DL is a low-current lamp to protect transistor ()1 from accidentalshorts in the battery or wrong polarity of the battery, by behaving as apositive temperature coefficient bias resistor or, in extreme cases, asa fuse.

Transformer T2 providing the reference voltage could be eliminated byproviding an extra secondary on transformer T1 for purposes ofestablishing the reference voltage.

While the control circuit illustrated is adapted for charging l2-voltbattkeries, the identical circuit could be used for lower batteryvoltage by lowering the secondary voltage of transformer T], andreadjusting the values of resistors R5 and R6 of the voltage dividernetwork. The circuitry could also be used for higher battery voltage by,for instance, raising the secondary voltage of transformer T1 andplacing a simple fixed voltage divider across the battery and theconnecting tap B of the protective lamp DL.

From the foregoing description and accompanying drawings it will be seenthat the invention provides an automatic battery-charging systemutilizing a phase shift control circuit to control triggering of anelectronic rectifier in the system so as to vary the power output of therectifier in inverse proportion to the charge on the battery.

The terms and expressions which have been used are used as terms ofdescription-and not of limitation, and there is no intention in the useof such terms and expressions of excluding having at least an anode, acathode, and a gate electrode,

said anode being connected to said source of current, said cathode beingadapted for connection in circuit with a battery,

a sensing circuit for sensing the potential on a battery and providing asensing signal when said potential is below a predetermined level,

means coupled to said sensing circuit for providing a predeterminedconstant reference voltage thereto, said sensing signal being producedwhen said reference vo|tage is greater than said predetermined level,

control means in circuit with said sensing circuit and with saidcontrolled electronic rectifier for triggering the conduction of saidcontrolled electronic rectifier in response to said sensing signal tocause charging of a battery at a rate inversely proportional to theamount of charge on the battery, and

current-limiting means in circuit with said controlled electronicrectifier for limiting the charging rate to a battery, said currentlimiting means comprising:

a transformer comprising a primary and a secondary, said primary beingin circuit with the cathode of said controlled electronic rectifier;

a potentiometer in circuit with the secondary of said transformer andcomprising a load therefor, said potentiometer determining the maximumpermissible current flow to a battery; and v transistor means in circuitwith said potentiometer and said control means for providing a signal tosaid control means to affect the conduction of said controlledelectronic rectifier in such a manner that the charging rate is reducedwhen the charging rate to a battery in response to said sensing signalwould otherwise be greater than said maximum permissible current flow.

'2. In a control circuit of the type which comprises a first source ofDC voltage, a controlled rectifier including an anode electrode and acathode electrode which define a current path I therebetween, saidcurrent path being in circuit with said first source, and agateelectrode for controlling the current fiow in said path, and abattery in circuit with said current path, the

conduction control means for causing said first transistor means toconduct when the charge on said battery is below said a a predeterminedlevel, said conduction control means comprising a source of referencepotential and variable resistance means in circuit with said source ofreference potential and said first transistor means,

gate control means in circuit with said first transistor means and saidgate electrode for controlling the average current flow in said currentpath in response to the magnitude of the sensing signal from saidtransistor means, said gate control means including second transistormeans in circuit with said first transistor means, charge storage meansin circuit with said second transistor means, wherein said secondtransistor partially determines the rate of charge and discharge of saidcharge storage means, and unijunction transistor means in circuit withsaid charge storage means, said resistive means and said gate electrodeof said controlled rectifier, said unijunction transistor meansproviding a control signal to said gate electrode in response to therate of charge and discharge of said charge storage means, and

current-limiting means in said current path for limiting the chargingrate to said battery, said current-limiting means comprising transformermeans in circuit with said controlled rectifier and fourth transistormeans in circuit with said transformer means and said unijunctiontransistor means for partially controlling the rate of charge anddischarge of said charge storage means to limit the charging rate tosaid battery.

3. A control circuit comprising the combination of: a first source of DCpotential which includes a first terminal and a second terminal, acontrolled rectifier including an anode electrode and a cathodeelectrode which define a current path therebetween said current pathbeing in circuit with said first terminal of said first source of DCpotential and a gate electrode for controlling the current flow in saidpath, and a battery including a first terminal in circuit with saidcurrent path and a second terminal in circuit with said second terminalof c said first source of DC potential, and control means forcontrolling the current flow in said current path to a rate which causescharging of the battery at a rate inversely proportional to the chargeon said battery, said control means including:

sensing means in circuit with said battery for sensing the charge onsaid battery, said sensing means including a first transistor which isbiased to be normally nonconductive and which is caused to conduct toprovide a sensing signal when the charge on said battery is below apredetermined level, means for causing said first transistor to conductat a predetermined level comprising a first source of referencepotential, and variable resistance means in circuit with said firstsource of reference potential and the base of said first transistormeans so that adjustment of said variable resistance means varies saidpredetermined level, gate control means in circuit with said firsttransistor means and said gate electrode for controlling the averagecurrent flow in said current path in response to the conduction of saidtransistor means, said gate control means including a second transistorin circuit with said first transistor for receiving said sensing signaland providing an output signal in response thereto, charge storage meansin circuit with said second transistor for receiving said output signal,resistive means in circuit with said charge storage means and saidsecond transistor for partially determining the rate of charge anddischarge of said charge storage means, and unijunction transistor meansin circuit with said charge storage means, said resistive means and saidgate electrode of said controlled rectifier for providing a controlsignal to said gate electrode which is responsive to the rate of chargeand discharge of said charge storage means, a first transformer couplingsaid first transistor to said second'transistor, and a second source ofDC potential isolated from said first source of DC potential, said firstsource of DC potential and said second source of DC potential beingderived from a common source of AC potential.

' .The ci'rcuit'as defined in claim 3 further including secondtransformer means for coupling said unijunction transistor means to saidgate electrode.

5. A circuit for charging a battery comprising: a source of currentincluding a source of AC potential, a

first transformer in circuit with said source of AC poten- I tial', anda full-wave rectifier in circuit with said first transformer,:

a control circuit including an electronic rectifier having an anode, acathode, and a gate electrode, said anode being in circuit with saidfull-wave rectifier, said cathode being 1 adapted for connection incircuit with a battery wherein said sensing circuit for sensing thecharge on a battery wherein said sensing means includes a firsttransistor which is normally biased to be nonconductive, and which iscaused to conduct when the potential on a battery falls below apredetermined level, the level of conduction of said first transistorbeing proportional to the amount by which the potential on a battery. isbelow the predetermined potential, said sensing circuit-furtherincluding a second transformer means in circuit with said source of ACpotential, and rectifier means for providing full-wave rectification ofthe signal at the output of the second transformer,

means. coupled to said sensing circuit for providing a predeterminedconstant reference voltage,

thirdv transformer means for coupling a control signal to said gateelectrode, and

control means in circuit with said sensing circuitand with saidrectifier through said third transformer means for automaticallycontrolling the conduction of said electronic rectifier to causecharging of a battery at a rate inversely proportional to the amount ofcharge on the battery.

6. The circuit as defined in claim 5 wherein said sensing circuitfurther comprises a voltage divider including a potentiometer in circuitwith said first transistor, said potentiometer being adapted for settingat a predetermined full-charge battery voltage so that said firsttransistor is nonconductive, and so that said first transistor conductswhen the reference voltage is higher than the battery voltage. i

7. The circuit as defined in claim 5 further including current-limitingmeans in circuit with said rectifier for limiting the charging rate to abattery. 8. The circuit' as defined in claim 5 wherein said controlmeans includes a phase shift circuit including 'a capacitor whichcontrols the signal applied to the gate electrode of said rectifier.

9. The circuit as defined in claim 8 wherein said control means furtherincludes a unijunction transistor in circuit with said capacitor andsaid gateelectrode of said rectifier.

10. The circuit as defined'in claim 5 wherein said sensing circuitfurther includes voltage divider means including spaced resistances anda potentiometer, said potentiometer being coupled to the base of saidfirst transistor and to said source of reference potential;

ll. The circuit as defined in claim 5 including means in said sensingcircuit for protecting the sensing circuit from damage due to reversingthe polarity of a battery applied to said sensing means.

12. The circuit as defined in claim 5 wherein said control meansincludes a secondtransistor, phase shifi means comprising a capacitor incircuit with said second transistor to provide a pulse for triggeringthe gate of said rectifier upon energization by said sensing circuit.

13. The circuit in accordance with claim 5 wherein said referencevoltage is maintained at a constant level by a Zener diode.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No.3,586,955 Dated June 22, 1971 Inventor(s) Edmund Kisiel It is certifiedthat error appears in the above-identified patent and that said LettersPatent are hereby corrected as shown below:

On the cover sheet [45] "June 15, 1971 should read PF June 22, 1971Signed and sealed this 18th day of April 1972.

(SEAL) Attest:

ROBERT GOTTSCHALK EDWARD M.FLETCHER,JR.

Commissioner of Patents Attesting Officer 1 FORM PC1-10 (1 USCOMM-DCsuave-pe U 5' GOVERNMENT PIUNTNG OFFICE 19.9 0*365-331

1. A circuit for charging a battery comprising: a source of current, acontrol circuit including a controlled electronic rectifier having atleast an anode, a cathode, and a gate electrode, said anode beingconnected to said source of current, said cathode being adapted forconnection in circuit with a battery, a sensing circuit for sensing thepotential on a battery and providing a sensing signal when saidpotential is below a predetermined level, means coupled to said sensingcircuit for providing a predetermined constant reference voltagethereto, said sensing signal being produced when said reference voltageis greater than said predetermined level, control means in circuit withsaid sensing circuit and with said controlled electronic rectifier fortriggering the conduction of said controlled electronic rectifier inresponse to said sensing signal to cause charging of a battery at a rateinversely proportional to the amount of charge on the battery, andcurrent-limiting means in circuit with said controlled electronicrectifier for limiting the charging rate to a battery, said currentlimiting means comprising: a transformer comprising a primary and asecondary, said primary being in circuit with the cathode of saidcontrolled electronic rectifier; a potentiometer in circuit with thesecondary of said transformer and comprising a load therefor, saidpotentiometer determining the maximum permissible current flow to abattery; and transistor means in circuit with said potentiometer andsaid control means for providing a signal to said control means toaffect the conduction of said controlled electronic rectifier in such amanner that the charging rate is reduced when the charging rate to abattery in response to said sensing signal would otherwise be greaterthan said maximum permissible current flow.
 2. In a control circuit ofthe type which comprises a first source of DC voltage, a controlledrectifier including an anode electrode and a cathode electrode whichdefine a current path therebetween, said current path being in circuitwith said first source, and a gate electrode for controlling the currentflow in said path, and a battery in circuit with said current path, theimprovement which comprises control means for controlling the currentflow in said current path to a rate which causes charging of the batteryat a rate inversely proportional to the charge on said battery, saidcontrol means including: sensing means in circuit with said battery forsensing the charge on said battery and providing a sensing signalindicative thereof, said sensing means including first transistor meanscapable of being rendered conductive to provide said sensing signal whenthe charge on said battery is below a predetermined level, conductioncontrol means for causing said first transistor means to conduct whenthe charge on said battery is below said a a predetermined level, saidconduction control means comprising a source of reference potential andvariable resistance means in circuit with said source of referencepotential and said first transistor means, gate control means in circuitwith said first transistor means and said gate electrode for controllingthe average current flow in said current path in response to themagnitude of the sensing signal from said transistor means, said gatecontrol means including second transistor means in circuit with saidfirst transistor means, charge storage means in circuit with said secondtransistor means, wherein said second transistor partially determinesthe rate of charge and discharge of said charge storage means, andunijunction transistor means in circuit with said charge storage means,said resistive means and said gate electrode of said controlledrectifier, said unijunction transistor means providing a control signalto said gate electrode in response to the rate of charge and dischargeof said charge storage means, and current-limiting means in said currentpath for limiting the charging rate to said battery, saidcurrent-limiting means comprising transformer means in circuit with saidcontrolled rectifier and fourth transistor means in circuit with saidtransformer means and said unijunction transistor means for partiallycontrolling the rate of charge and discharge of said charge storagemeans to limit the charging rate to said battery.
 3. A control circuitcomprising the combination of: a first source of DC potential whichincludes a first terminal and a second terminal, a controlled rectifierincluding an anode electrode and a cathode electrode which define acurrent path therebetween said current path being in circuit with saidfirst terminal of said first source of DC potential and a gate electrodefor controlling the current flow in said path, and a battery including afirst terminal in circuit with said current path and a second terminalin circuit with said second terminal of said first source of DCpotential, and control meanS for controlling the current flow in saidcurrent path to a rate which causes charging of the battery at a rateinversely proportional to the charge on said battery, said control meansincluding: sensing means in circuit with said battery for sensing thecharge on said battery, said sensing means including a first transistorwhich is biased to be normally nonconductive and which is caused toconduct to provide a sensing signal when the charge on said battery isbelow a predetermined level, means for causing said first transistor toconduct at a predetermined level comprising a first source of referencepotential, and variable resistance means in circuit with said firstsource of reference potential and the base of said first transistormeans so that adjustment of said variable resistance means varies saidpredetermined level, gate control means in circuit with said firsttransistor means and said gate electrode for controlling the averagecurrent flow in said current path in response to the conduction of saidtransistor means, said gate control means including a second transistorin circuit with said first transistor for receiving said sensing signaland providing an output signal in response thereto, charge storage meansin circuit with said second transistor for receiving said output signal,resistive means in circuit with said charge storage means and saidsecond transistor for partially determining the rate of charge anddischarge of said charge storage means, and unijunction transistor meansin circuit with said charge storage means, said resistive means and saidgate electrode of said controlled rectifier for providing a controlsignal to said gate electrode which is responsive to the rate of chargeand discharge of said charge storage means, a first transformer couplingsaid first transistor to said second transistor, and a second source ofDC potential isolated from said first source of DC potential, said firstsource of DC potential and said second source of DC potential beingderived from a common source of AC potential.
 4. The circuit as definedin claim 3 further including second transformer means for coupling saidunijunction transistor means to said gate electrode.
 5. A circuit forcharging a battery comprising: a source of current including a source ofAC potential, a first transformer in circuit with said source of ACpotential, and a full-wave rectifier in circuit with said firsttransformer, a control circuit including an electronic rectifier havingan anode, a cathode, and a gate electrode, said anode being in circuitwith said full-wave rectifier, said cathode being adapted for connectionin circuit with a battery wherein said sensing circuit for sensing thecharge on a battery wherein said sensing means includes a firsttransistor which is normally biased to be nonconductive, and which iscaused to conduct when the potential on a battery falls below apredetermined level, the level of conduction of said first transistorbeing proportional to the amount by which the potential on a battery isbelow the predetermined potential, said sensing circuit furtherincluding a second transformer means in circuit with said source of ACpotential, and rectifier means for providing full-wave rectification ofthe signal at the output of the second transformer, means coupled tosaid sensing circuit for providing a predetermined constant referencevoltage, third transformer means for coupling a control signal to saidgate electrode, and control means in circuit with said sensing circuitand with said rectifier through said third transformer means forautomatically controlling the conduction of said electronic rectifier tocause charging of a battery at a rate inversely proportional to theamount of charge on the battery.
 6. The circuit as defined in claim 5wherein said sensing circuit further comprises a voltage dividerincluding a potentiometer in circuit with said first transistor, saidpotentiometer being adapTed for setting at a predetermined full-chargebattery voltage so that said first transistor is nonconductive, and sothat said first transistor conducts when the reference voltage is higherthan the battery voltage.
 7. The circuit as defined in claim 5 furtherincluding current-limiting means in circuit with said rectifier forlimiting the charging rate to a battery.
 8. The circuit as defined inclaim 5 wherein said control means includes a phase shift circuitincluding a capacitor which controls the signal applied to the gateelectrode of said rectifier.
 9. The circuit as defined in claim 8wherein said control means further includes a unijunction transistor incircuit with said capacitor and said gate electrode of said rectifier.10. The circuit as defined in claim 5 wherein said sensing circuitfurther includes voltage divider means including spaced resistances anda potentiometer, said potentiometer being coupled to the base of saidfirst transistor and to said source of reference potential.
 11. Thecircuit as defined in claim 5 including means in said sensing circuitfor protecting the sensing circuit from damage due to reversing thepolarity of a battery applied to said sensing means.
 12. The circuit asdefined in claim 5 wherein said control means includes a secondtransistor, phase shift means comprising a capacitor in circuit withsaid second transistor to provide a pulse for triggering the gate ofsaid rectifier upon energization by said sensing circuit.
 13. Thecircuit in accordance with claim 5 wherein said reference voltage ismaintained at a constant level by a Zener diode.